The invention relates to an improved crystal holder for mixer diode such as Schottky diode of the wafer type operating in the millimeter band. Semiconductive diodes are used in mixer or frequency converters in communication systems operating in the millimetric range. Waveguide is the usual transmission line used at such frequencies. The crystal holder is to be designed so as to fit in the waveguide. A type of crystal holder for waveguide circuits has been described by Sharpless in Bell System Technical Journal Volume 25, page 1385 (1956) in the article entitled "Wafer type millimetric wave rectifiers."
In the Transactions of IEEE on Microwave Theory and Techniques, Volume 16, No. 5, issued in May 1968, a crystal holder of the Sharpless type is described to be used in a circuit operating at a carrier frequency of 51.7 GHz and an intermediate frequency of 1.3 GHz. FIG. 1 taken from the above article shows the crystal holder made of a metal wafer 1 machined with a rectangular hole 2 the smallest dimension of which is equal to the smallest dimension of waveguide type RG 97/U while the largest dimension is larger than the corresponding waveguide dimension. The wafer is also machined with a circular aperture 3 and a recess 4 for the central conductor 5 of a standard coaxial plug which constitutes the intermediate frequency output. Conductor 5 is connected to a wire 6 which is bent at 90.degree. in order to penetrate in the holes 7 and 8 machined in the thickness of plate 1. Wire 6 is set in holes 7 and 8 by means of dielectric sheaths. The end of wire 6 terminates at the level of the inner wall of hole 2 and contacts the Schottky chip which is not visible in FIG. 1.
It is known that a high modulation yield requires that the local frequency of carrier frequency wave should not be transmitted in the intermediate frequency amplifier as explained in the Bell System Technical Journal, article published page 1291 of the September 1968 issue.
The main object of the present invention is a crystal holder which provides rejection of the carrier or local frequency and impedance matching at the intermediate frequency that is, the impedance at plug 5 is 50 ohms or any other standardized value. The crystal holder according to the invention provides the following advantages. Impedance matching at intermediate frequency increases the conversion yield with respect to prior art by at most one decibel. The diode seen from the input of the crystal holder has an impedance equal to 50 ohms. It is therefore possible to interchange any crystal holder without any further adjustment of the intermediate frequency circuit. Individual matching of crystal holder with respect to individual diode is performed in the factory through control of the interconnecting wire length and diameter. Experience shows that for a given batch of diodes the same mechanical setting can be used.